Known air distillation apparatus comprise a main heat exchanger for cooling down feed air which has been compressed, cooled and freed of impurities; a rectification column comprising a rectifying portion for separating the thus-cooled feed air introduced here to an oxygen-enriched component and a nitrogen component, and a condenser for partially condensing the separated nitrogen component to provide a reflux liquid; a liquid nitrogen storage tank for supplying liquid nitrogen to said rectification column as a part of the reflux liquid and a source of cold by way of a supply valve; and a cold supply route for supplying cold to said main heat exchanger.
In such a unit, nitrogen gas is mainly produced, for instance, by compressing air taken in from the atmospheric air by a compressor, cooling down said air by a refrigerator, and removing impurities such as carbon dioxide and moisture therefrom by an adsorption device or the like, and then cooling down this feed air nearly to its liquefying point by utilizing cold of waste gas in a main heat exchanger, and introducing the thus-cooled feed air to a rectification column, separating it to an oxygen-enriched component and a nitrogen component in the rectifying portion of said rectification column, and partially condensing the separated nitrogen component in a condenser to provide a reflux liquid, and on the other hand supplying liquid nitrogen from a liquid nitrogen storage tank to said rectification column as a part of the reflux liquid and a source of refrigeration by way of a supply valve.
Although an oxygen-enriched liquid reserved in the bottom of the rectification column is transported as refrigeration to the condenser and reserved in the same condenser, in the aforementioned unit, it is required, for stable rectification in the rectifying portion even when the consumption of a product gas varies, that the height of a liquid level of said oxygen-enriched liquid reserved in the condenser is made almost constant so as to make the condensing capacity (the cooling capacity) of said condenser placed in the upper part of the same rectifying portion or outside of the rectification column almost constant.
Known methods of controlling the liquid level in a condenser comprise:
(1) In J-A-61046747, there is proposed a method of regulating the amount of liquid nitrogen assist to be supplied as a part of a reflux liquid and a source of refrigeration, in dependence on liquid level in a condenser, without regulating the amount of liquefied air reserved in the bottom of a rectification column and introduced into the condenser. PA1 (2) In J-A-64054187, there is proposed a method of detecting the pressure of product nitrogen gas, and regulating the amount of liquefied air (an oxygen-enriched component) reserved in the bottom of a rectification column introduced into a condenser and the amount of the product nitrogen gas. This method causes the liquid level in the condenser to vary, thereby coping with the variation in the consumption of product nitrogen gas, but it is not possible to keep the liquid level of cold in the condenser almost constant so that the condensing capacity (the cooling capacity) of the condenser is made almost constant.
In method of J-A-61046747, however, the flow rate of liquefied air in a conduit from the bottom of a rectification column to a condenser seldom varies, even if the supply of liquid nitrogen supplied as a part of a reflux liquid and a source of cold is regulated because liquefied air is reserved in the bottom of a rectification column, and as a result, control gets impossible of being followed up to the change of the liquid level of cold in the condenser and hence the constancy of rectification can not be kept by keeping the liquid level of cold in the condenser almost constant. Even when the supply of liquid nitrogen is regulated by the control of the method of J-A-61046747 and the amount of liquid air flowing down to the bottom of the rectification column is changed by virtue of this regulation, the reserve amount of liquefied air reserved in the bottom of the rectification column merely varies and the flow rate thereof in said route seldom vary, and hence cold even in an amount only of compensating for the change of the liquid level of cold in the condenser is not supplied, because the flow rate of liquefied air in a conduit leading it from the bottom of a rectification column to a condenser will be determined depending on the pressure in the bottom of the rectification column, the pressure in the condenser or the opening degree of a valve provided in the conduit between them. As a result, control becomes impossible of being followed up to the change of the liquid level in the condenser, and in an extreme case, the condenser becomes empty or completely full.
In a case where oxygen gas is produced, on the other hand, a duplex rectification column is generally used comprising: a medium-pressure rectification column having a medium-pressure rectifying portion for separating feed air which has been cooled as in the aforementioned case, introduced here, to an oxygen-enriched component and a nitrogen component, and a condenser for condensing the separated nitrogen component to provide a reflux liquid; a low-pressure rectification column having a low-pressure rectifying portion for using as a reflux liquid a part of the reflux liquid in said medium-pressure rectification column introduced here by way of an expansion valve, and separating the oxygen-enriched component introduced here from the bottom of said medium-pressure rectification column to an oxygen component and a nitrogen component, and a cold reserving portion of said condenser of allowing said oxygen component to flow in from the same low-pressure rectifying portion; a liquid oxygen storage tank for supplying liquid oxygen to the same cold reserving portion by way of a supply valve; and a cold supply route for supplying cold to a main heat exchanger. Even in the condenser of said medium-pressure rectification column, however, there will be easily caused problems similar to the aforementioned case.